The 'Altmark Incident' was a naval episode in which British destroyers and the German tanker Altmark (16/17 February 1940).

The episode took place in what were, at that time, neutral Norwegian waters. On board Altmark, captained by Heinrich Dau, were some 300 Allied prisoners (officially internees), whose ships had been sunk by the pocket battleship Admiral Graf Spee in the course of her commerce-raiding foray the southern parts of the Atlantic and Indian Oceans.

British naval forces cornered the tanker, and a party from the destroyer Cossack later boarded Altmark near the Jøssingfjord and liberated all the prisoners. Eight German sailors were killed and 10 others wounded during the hand-to-hand fighting which took place during the boarding; one British sailor was also wounded during the fighting. The German government claimed that the boarding was a violation of international law and Norwegian neutrality, and later used the incident in the propaganda broadcasts of 'Lord Haw-Haw'.

In February 1940, Altmark was returning to Germany with 299 captured British sailors on board. These men were prisoners of war who had been recovered from merchant vessels sunk by Admiral Graf Spee, for which Altmark was serving as a tanker and supply vessel. On her passage from the southern part of the Atlantic to Germany, Altmark passed through Norwegian waters: international law did not ban the transfer of prisoners of war through neutral waters. On the insistence of British contacts who had been pursuing the vessel, Altmark was investigated three times on 15 February by vessels of the Norwegian navy. The German ship was initially boarded by officers from the torpedo boat Trygg off Linesøya, then by officers from the torpedo boat Snøgg in the Sognefjord, and finally personally by Kontreadmiral Carsten Tank-Nielsen and naval personnel from the destroyer Garm in the Hjeltefjord. In each instance, the men who boarded the ship carried out cursory searches and took the Germans' word that the vessel was conducting purely commercial business. The British prisoners held in the ship’s hold reportedly made strenuous efforts to signal their presence, but the Norwegian search parties did not inspect the hold, and allowed the ship to continue on her way.

After the third boarding, Altmark was escorted to the south by the torpedo boats Skarv and Kjell and the guard boat Firern.

Altmark was sighted off Egersund later on the same day by British aircraft, which raised the alarm in the Royal Navy. The aircraft were stationed at RAF Thornaby, in north-eastern England. After being intercepted by the destroyer Cossack, commanded by Captain Philip Vian, Altmark sought refuge in the Jøssingfjord, but Cossack followed her into these waters on the next day. Altmark's Norwegian naval escorts blocked the initial British attempts to board the ship, and trained their torpedo tubes on Cossack. Vian then asked the Admiralty for instructions, and received the following orders directly from the First Lord of the Admiralty, Winston Churchill: 'Unless Norwegian torpedo-boat undertakes to convoy Altmark to Bergen with a joint Anglo-Norwegian guard on board, and a joint escort, you should board Altmark, liberate the prisoners, and take possession of the ship pending further instructions. If Norwegian torpedo-boat interferes, you should warn her to stand off. If she fires upon you, you should not reply unless attack is serious, in which case you should defend yourself, using no more force than is necessary, and ceasing fire when she desists. Suggest to Norwegian destroyer that honour is served by submitting to superior force.'

The British government made no particular objection to the fact of a prison ship traversing neutral waters. In fact, in official papers regarding the incident, they noted that the Royal Navy had done the same, for example in December 1939, when the light cruiser Despatch passed through the Panama Canal, which was neutral waters, with German prisoners from the freighter Düsseldorf. But Altmark's crew had gone hundreds of miles out of their way to make the long run through Norwegian waters to Germany. Besides, the Norwegian government had not permitted the Germans to transport prisoners through Norwegian waters, Altmark having falsely claimed to be carrying no such persons, nor had the crew been truthful regarding the nature of their cargo and voyage.

The Norwegian naval forces refused to take part in a joint escort, reiterating that their earlier searches of Altmark had found nothing. Vian then stated that he intended to board Altmark and invited the Norwegians to take part, but again they demurred. In the ensuing incident, Altmark ran aground, and the British then boarded her at 22.20 on 16 February and, after some hand-to-hand fighting with bayonets. overcame the German ship’s crew and went down to the hold.  One of the released prisoners stated that the first they knew of the operation was when they heard the shout 'Any Englishmen here?' from the boarding party. When the prisoners shouted back "Yes! We are all British!', the response was 'Well, the Navy’s here!' (German official documents mention 303 internees while the British reported that they had liberated 299.)

The German dead were buried in Sogndal Cemetery above Jøssingfjord.  Cossack left the Jøssingfjord just after midnight on 17 February. The Norwegian escorts protested, but did not intervene. The official explanation later given by the Norwegian government was that, according to international treaty, a neutral country was not obliged to resist a vastly superior force.

The Norwegians were angered that their neutrality had been infringed, as they wished not to be dragged into the war. Nonetheless, the 'Altmark Incident' sowed doubts about Norwegian neutrality among the Allies and in Germany. Adolf Hitler, who had earlier decided on 14 December 1939 for the invasion of Norway after discussions with Admiral Erich Raeder and the Norwegian Nazi leader Vidkun Quisling, ordered intensified planning on 19 February 1940 for attacks on Norway and Denmark, which eventually took place on 9 April 1940 as 'Weserübung'.

The 'Altmark Incident' gave the British a short-lived but sorely needed morale boost during the period of the 'Phoney War'. The incident also had a lasting propaganda effect in German-occupied Norway during the war, when the Norwegian collaborationist government tried to neutralise their nickname 'quislings' by using the location of the skirmish, Jøssingfjord, to coin the derogatory term 'jøssing' for those of pro-Allied and anti-Nazi sentiment. Their efforts backfired, as 'jøssing' was immediately adopted as a positive term by the general public, and the word had been banned from official use by 1943.